This invention was made with Government support under Grant No. R01 GM 39803, awarded by the National Institute of Health. The Government has certain rights in the invention. The present invention relates to apparatus for guiding X-rays along the hollow bore of a tapered glass capillary, and more particularly to a method and apparatus for securing such a capillary so that it is sufficiently straight to propagate X-rays.
Edward A. Stern et al, in an article entitled "Simple Method for Focusing X-Rays Using Tapered Capillaries", Applied Optics, Vol. 27, No. 24, Dec. 15, 1988, pages 5135 to 5139, provided a thorough analysis of the method of focusing X-rays through the use of tapered capillaries. As pointed out by Stern et al, for many uses of X-rays it is necessary or desirable to focus them into a very small spatial region. The standard methods for doing this require very precise dimensions in the focusing elements, on the order of microns or less, and consequently such focusing typically has been difficult and expensive. As described in the Applied Optics article, however, X-rays can be focused by the use of a capillary which has an entrance opening having the dimension of the incident X-ray beam and having an exit opening which has the dimension which is desired for the focused beam. The X-ray beam which is to be focused is directed into a capillary so that the rays impinge on the inner surface of the capillary wall at angles below the critical glancing angle and reflect from that inner surface due to total external reflection so that the capillary acts as a waveguide. By appropriately narrowing the capillary along its length, the X-rays are concentrated over a broad band of energies so that the X-rays which pass through the central aperture of the capillary are, in effect, focused when they pass out the small end of the capillary, since the cross-sectional dimension of the beam is reduced and its intensity is increased.
Very short, rapidly tapering glass capillaries may be formed, as by drawing, to produce elongated glass tubes having an internal surfaces which taper inwardly from their inlet ends to their outlet ends for use in aperture visible light, as described in U.S. Pat. No. 4,917,462 to Isaacson et al. Because of critical angle limitations for X-rays, however, capiillaries fabricated for X-ray focusing must be very different than those used for aperturing visible light.
In the Applied Optics article, an untapered glass capillary is used to provide intensity enhancement of X-rays, and this capillary rested on a coextensive support, such as a V-groove formed in a metal plate. According to that article, a tapered capillary needs to be several meters long in order to concentrate a 500 micrometer diameter beam to a spot 10 micrometers in diameter. It has been found that the provision of a tapered glass tube of this length creates problems in handling, and that resting such a capillary on a metal plate does not maintain its linearity within necessary tolerances to maximize the output of X-rays. Although capillaries are inexpensive and simple to fabricate, and do not require the extreme precision of dimensions or shape necessary with the methods of focusing utilizing mirrors and zone plates currently in use, nevertheless it has been found that it is necessary to maintain such a capillary linear within a fraction of a milliradian of resolution to prevent absorption of the X-rays by the glass wall as they propagate along the capillary bore. Furthermore, because the glass tube is fragile, it is desirable to provide a coating on the exterior surface of the tube to give it strength and flexibility. However, such a coating can interfere with the ability to maintain the linearity of the capillary.